Electric resistance heating element

ABSTRACT

An electric resistance heating element for a moving air system in which the element is composed of a thin strip of apertured, foil-like material formed with a series of continuous and generally parallel extending corrugations which establish a wavy pattern. The foil is supported by a central, longitudinally extending, electrically insulated rod.

United' States Patent Fedor [45] Mar. 21, 1972 [541 ELECTRIC RESISTANCEHEATING ELEMENT r [56] References cued [72] Inventor: Robert J Fedor,Westlake, Ohio UNITED STATES PATENTS [73] Assignee: Gould lne., Chicago,111. 1,006,655 10/1911 Harris ..338/291 1,860,493 5/1932 Campbell..338/287 [221 F1|ed= 1971 1,991,935 2/1935 Melsom ..338/333 x [2!] App!Nu; 129,893 3,244,860 4/1966 Lindley ..219/374 m s ti n Data PrimaryExaminer-C. L. Albritton Att0rney-Edward E. Sachs [63]Continuation-impart of Ser. No. 96,322, Dec. 9, 1970, 1

abandoned. I [57] g ABSTRACT An electric resistance heating element fora moving air system 2 [5 l U 8 CI 9 9 219/375 47 in which the element iscomposed of a thin strip of apertured, 51 I t Cl "95b foil-like materialformed with a series of continuous and E i o'f'seirch 21 9 l 2 3 53 355generally parallel extending corrugations which establish a "saw wavypattern. The foil is supported by a central, longitudinally extending,electrically insulated rod.

9 Claims, 3 Drawing Figures ,vaslyiv ELECTRIC RESISTANCE HEATING ELEMENTThis application is a continuation in part application of US.Application, Ser. No. 96,322, filed Dec. 9, 1970 and now abandoned.

The present invention relates generally to an electric resistanceheating element and, more particularly, to an element which is composedof a thin strip of foil-like material and is adapted to be employed inmoving air systems.

In the prior art electric resistance heating elements are commonlyconstructed by either utilizing a self-supporting expanded meta] stripwhich is secured between two supporting members, or by using a coiledwire arrangement which is supported at intermediate locations by meansof a ceramic bearing mount which surrounds the wire.

The conventional coiled wire construction can be characterized as havinga low surface areaand a high mass, or a low surface area to mass ratio.The high mass leads to a relatively high raw material cost, while thelow surface area leads to inefficient heating and cooling. As aconsequence of the high raw material cost, a constant effort is made tominimize the amount of material used by deliberately operating theelement at as high a temperature as possible, usually well into the redheat range. The operation of the element at the high tempera ture levelhas at least two detrimental consequences. The areas of low air flow(inside the ceramic bushings) become overheated and are prone tofailure; secondly, the efficiency of convective air heating is decreasedas a significant amount of the energy is spent in radiant heating thesolids surrounding the element. The low surface area to mass ratio alsocauses a slow heat-up and cooling rate.

In the past, considerable experimentation and effort has been expendedto produce an element with a high surface area to mass ratio. However,to date, no economical method has been found to provide a system whichwould have the desired low weight and high surface area without theassociated problem of excessive sagging.

It is the primary object of the present invention to provide an electricheating element which overcomes the disadvantages common in the priorart.

It is a further main object of the present invention to provide anelectric resistance heating element which is very light weight, yet issupported in a manner that will prevent sagging.

It is a further object of the present invention to provide a heatingelement having an expanded or otherwise apertured metal foil grid whichprovides at least the same electrical characteristics as prior artdevices; however, with a considerable saving in raw material.

It is another object of the present invention to provide a heatingelement which lends itself to assembly by automated means.

It is another object of the present invention to provide a heatingelement in which the support for the heating element is internal ratherthan external, as is the common practice today.

It is another object of the present invention to provide a heatingelement which is composed of corrugations establishing a wavy patternwhich has the advantage of placing a long length of material in arelatively small area to obtain a relatively high electric resistance.

It is another object of the present invention to provide a heatingelement in which the heating element establishes a corrugatedand wavypattern which is supported at very short intervals to prevent, or aleast to substantially reduce, creeping or sagging.

An aspect of the present invention resides in the provision of anelectric resistance heating element for a moving, or forced air systemwhich includes a thin strip of apertured foillike, electric resistantmaterial which is formed with a series of continuous and generallyparallel extending corrugations which establish a wavy pattern. Asupporting rod having an electrically non-conductive surface extendsthrough most of the corrugations and is effective to structurallysupport the strip at very short intervals. A structural frame isprovided to connect to the support rod for mounting the assembly of therod and the strip.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

In the drawing:

FIG. 1 is a perspective view of a heating element in accordance withthis invention;

FIG. 2 is a plan view of the heating element shown in FIG. 1; and

FIG. 3 is an elevational side view of the heating element.

Referring now to the drawing there is shown a foil-like strip 10 ofcommercially available material which has electric resistancecharacteristics. As the term foil" is used herein, it is to denotematerials having a thickness of between 4 to 16 mils (inch). While theinvention is, theoretically, utilizable for strips having a thicknessgreater than 16 mils (inch), it should be noted, however, that as thethickness of the foil-like strip increases, the weight increasescorrespondingly and thus becomes, at a given point, uneconomical.Similarly, as the foil thickness decreases below 4 mils, the cost of theraw material increases significantly to the point where the use of theelement as an electric resistance member becomes uneconomical.Conventional heating elements, by comparison, usually are composed ofwire or sheet material having a thickness of 40 to 60 mils.

The heating element 10, as shown in the drawing, has a diamond shapeexpanded metal configuration. The expansion of the metal is accomplishedby first splitting the solid foil strip intermittently so that theentire sheet has a series of closely spaced parallel cuts, to permitexpanding it laterally to form the open screen. However, the inventionis not limited to expanded metal elements. For example, the strip orelement l0 can be composed of a solid foil which is mechanicallyperforated by impact, or wherein the apertured foil is electrochemicallyformed.

While the diamond shape configuration, as shown at 12 in the drawing, isusually made by an expanded metal process, such configuration is notnecessarily the most desirable. For instance, it has been found that arectangular or square-like grid pattern has a more direct current pathand thus is preferred from an electrical performance point of view;however, such configuration does present cost and formability problemsin the manufacture thereof.

The strip 10 is formed with a series of continuous and generallyparallel extending corrugations 14 which establish in their totality awavy pattern. While there are shown two rows of such heating elements,it will be appreciated that numerous longitudinally extending rows,which are electrically connected in series, can be constructed; all ofsuch rows forming together a continuous loop with one end of the loopbeing connected to a current terminal 16 and the other end beingphysically and electrically connected to the other electric terminal 18.The terminals 16 and 18 are suitably mounted to a common plasticinsulating mount 20.

In order to support the strip 10, an particularly the individualcorrugations 14 thereof, there is provided a longitudinally extendingrod 22 which protrudes through the individual corrugations in askewer-like manner. The rod 22 extends through the corrugations 14 abouta central axis to provide maximum support for the corrugations 14 whichare also generally symmetrically arranged along and parallel to thelongitudinal axis of the rod. The rod 22 has a dielectric strength of atleast 2% thousand volts. This is typically accomplished by providing asteel rod coated with ceramic material, although a solid glass or quartzrod may be utilized. Alternatively, a steel rod having a ceramic sleevehas also been found to be satisfactory.

The assembly of the rod 22 and the corrugated strip structure l0, 14 issupported by means of a simple frame which comprises a network of rods24 connecting at opposite ends to the insulated rods 22 and to amounting plate 26.

The present invention will be better understood when the operatingcharacteristics are compared with those of the conventional devices. Forexample, a typical 5,600 watt laundry heating element of standard coiledwire construction has the following characteristics:

Surface Area 69 in. Weight 0.265 Surface Area to Mass Ratio 260 in./#Room Temperature Resistance 9.3 ohms Operating Temperature at 236 voltsnominal l600l800 F. Alloy Type C Wire Gage l6=(.0$08") Time to dry 10load of towels 70 minutes Length of Wire 35.6 ft.

The Type C resistance alloy, with a nominal composition of6ONi-24Fe-16Cr, is commonly used for this application because it hassufficient oxidation resistance at the operating temperatures with aminimum utilization of expensive nickel. On the other hand, a Type Dalloy, with a composition of 35Ni-45Fe-20Cr, would be less expensivebecause of the lesser amount of nickel; however, its elevatedtemperature properties are not conducive to a long operating life.Typically, when failure of the wire does occur, the break is in an areawhere air blockage (such as inside a ceramic bushing) causes thetemperature of wire to exceed the temperature capability of the wire.

As a comparative specific example of the operating characteristics ofthe high surface area to mass ratio heating element in accordance withthe invention, the performance data of a 5,600 watt laundry type heatingelement can be described as:

Surface Area 96 in. Weight 0.100 Surface Area to Mass Ratio 960 in./#Room Temperature Resistance 9.3 ohms Operating Temperature Black to lS50F.

(236 volt nominal) Alloy Type C Thickness 0.0!2" Pattern Diamond shapedapertures .50 in. in length .20 in. in width .0l8 in. in strand widthLength of strip ll ft. Time to dry [0 load of towels 54 min.

shown that the desirable ratio range is approximately 900 to 2,500 in?/#When the upper end of the ratio range is appreciably exceeded, economicconsiderations such as excessive cost of material and fabricatingexpenses nullify or depreciate the advantages of a thin foil heatingelement. Similarly, when the lower end of the ratio range is exceeded,the desired performance characteristics are significantly lost.

In addition, the skewered thin foil construction does not have areas ofair blockage analagous to the bushing areas of the coiled wire design.And the operating temperature can be maintained considerably below theupper limit of performance of the Type C alloy, thus permitting the useof less expensive alloys such as, for example, Type D (35Ni-45 Fe-20Cr).

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention and it is aimed, thereforein the appended claims to cover all such changes and modifications asfall within the true spirit and scope of the invention.

What is claimed is:

1. An electric resistance heating element particularly for moving airsystems, comprising:

a thin strip of apertured foil-like, electric resistance, materialformed as a grid with a series of continuous and generally parallelextending corrugations establishing a wavy pattern;

a support rod with an electrically nonconductive surface extendingthrough most of said corrugations, effective to structurally supportsaid strip at short intervals; and

frame means connecting to said support rod for mounting the assembly ofsaid rod and strip.

2. An electric resistance heating element according to claim 1, whereinsaid rod extends through said corrugations about a central axis.

3. An electric resistance heating element according to claim 1, whereinsaid corrugations are generally symmetrical and extend along alongitudinal axis.

4. An electric resistance heating element according to claim 1, whereinsaid corrugations form a continuous loop of said strip of foil-likematerial.

5. An electric resistance heating element according to claim 1, whereinthe apertures of the foil-like material are symmetrically arrayed.

6. An electric resistance heating element according to claim 1, whereinsaid material is electrochemically formed.

7. An electric resistance heating element according to claim 1, whereinsaid material is an expanded metal foil grid.

8. An electric resistance heating element according to claim 1, 'whereinsaid corrugations are supported on said rod in a skewer-like fashion.

9. An electric resistance heating element according to claim 1, whereinsaid foil has a thickness approximately in the range of4 to 16 milsinch.

1. An electric resistance heating element particularly for moving airsystems, comprising: a thin strip of apertured foil-like, electricresistance, material formed as a grid with a series of continuous andgenerally parallel extending corrugations establishing a wavy pattern; asupport rod with an electrically nonconductive surface extending throughmost of said corrugations, effective to structurally support said stripat short intervals; and frame means connecting to said support rod formounting the assembly of said rod and strip.
 2. An electric resistanceheating element according to claim 1, wherein said rod extends throughsaid corrugations about a central axis.
 3. An electric resistanceheating element according to claim 1, wherein said corrugations aregenerally symmetrical and extend along a longitudinal axis.
 4. Anelectric resistance heating element according to claim 1, wherein saidcorrugations form a continuous loop of said strip of foil-like material.5. An electric resistance heating element according to claim 1, whereinthe apertures of the foil-like material are symmetrically arrayed.
 6. Anelectric resistance heating element according to claim 1, wherein saidmaterial is electrochemically formed.
 7. An electric resistance heatingelement according to claim 1, wherein said material is an expanded metalfoil grid.
 8. An electric resistance heating element according to claim1, wherein said corrugations are supported on said rod in a skewer-likefashion.
 9. An electric resistance heating element according to claim 1,wherein said foil has a thickness approximately in the range of 4 to 16mils inch.